This invention relates generally to the field of electrodeposition of zinc or zinc alloys from electroplating baths. More particularly, this invention relates to the electrodeposition of zinc or zinc-nickel alloys from an electroplating bath to which has been added a corrosion inhibitive pigment.
Zinc is one of the most widely used metallic coatings for steel surfaces to protect such surfaces from corrosion. Two widely practiced methods of applying zinc to a steel surface is hot dip coating and electroplating. The former produces relatively heavy coatings and the surface thereof is generally characterized by a spangled finish. An electroplated coating, on the other hand, is relatively thin with a smooth bright surface which may be painted. Additionally, the latter coating may be formed to a more drastic degree than a hot dip coating without adversely affecting the adhesion of the coating to the ferrous base.
Over the years there have been many attempts to improve the properties of an electroplated product, particularly the corrosion resistant properties of a zinc electroplated steel. To this end workers in the art began to look at zinc-alloy coatings. For example, in U.S. Pat. No. 2,419,231, an electroplating bath capable of depositing an alloy of 85% zinc and 15% nickel is disclosed. On the other hand, U.S. Pat. No. 3,420,754 reported on certain problems associated with the practice taught in the above patent. The answer, among other changes, was the adoption of a lesser and more restrictive nickel range for the alloy. While such a product possessed better corrosion resistance than a single metal coating, such as zinc, there were problems in producing the alloy coatings.
A major effort in the field of electroplating was in the use of additives to the electroplating bath to improve the properties of the coated product. For example, U.S. Pat. No. 4,249,999 teaches a method of electrolytically plating a steel strip with a zinc-nickel alloy by including in the electroplating bath a strontium compound. Heretofore one of the difficulties in electroplating zinc alloys was due to the presence of contaminants in the plating bath. For example, rather than producing a product having a bright finish, the finish was dull or at best an uneven brightness. This appearance was due to unavoidable fluctuations in plating conditions, bath temperature, bath composition, and changes in the pH of the bath, caused by the presence of contaminants in the bath. As a consequence, commercial zinc-nickel electroplated products were slow in developing.
U.S. Pat. No. 4,251,329 discloses a process for improving the corrosion resistant properties of a zinc-nickel electroplated product by the step of performing the electroplating operation in a bath containing a vanadium compound along with the zinc and nickel. Exemplary of a vanadium compound is vanadyl sulfate.
Attempts to improve the corrosion resistant properties of single metal electroplated products has also been practiced by the use of additives to the electroplating bath. In this regard see U.S. Pat. Nos. 4,064,320; 4,048,381; and 3,986,843. All such patents relate to zinc-iron based acidic electroplating processes in which one or more additives are selected from the group consisting of Cr.sup.3+, Cr.sup.6+, zirconium, tin and indium.
None of these efforts met with commercial success for one or more of a variety of reasons, many of which were confirmed during the investigation leading to this invention. Details of such investigations and the results thereof will be found in the specifications which follow.